1. Field of the Invention
The present invention relates to content distribution systems and, more particularly, to a method and apparatus for delivering compressed video to subscriber terminals.
2. Description of the Background Art
Multimedia distribution systems are becoming increasingly important vehicles for delivering video, audio and other data (generally referred to as content streams) to and from remote users. Notably, switched digital video (SDV) systems have been developed to deliver content streams to subscribers over limited bandwidth transmission networks. Such transmission networks include, for example, digital subscriber line (DSL) networks, fiber-to-the-curb (FTTC) networks, and fiber-to-the-home (FTTH) networks. SDV systems typically distribute content streams using a packet-based transmission protocol, such as asynchronous transfer mode (ATM), transmission control protocol/internet protocol (TCP/IP), and the like, as well as combinations of such protocols (e.g., TCP/IP encapsulated by ATM). Subscribers receive the packetized streams via the appropriate termination equipment (e.g., DSL modems).
Typically, the number of channels for content service transmission that are supported by the transmission network is less than the total number of content streams accessible by the SDV system. Thus, the SDV system is configured to switch subscriber-desired content streams among the available channels supported by the transmission network. To change streams, a subscriber transmits a request to the network for the new stream and the network multiplexes the requested stream onto one of the available channels supported by the transmission network.
There are various factors that affect the channel-change time experienced by a subscriber requesting a channel-change. One of the largest contributors to the delay in acquiring a new stream in response to a channel-change is the decoder's dependence on receiving an independent reference frame in the content stream in order to produce decoded content. For example, a compressed video stream, such as a moving picture experts group (MPEG) stream, includes independent reference frames, referred to as intra-coded frames (I-frames), as well as predicted frames (P-frames) and bi-directionally interpolated frames (B-frames). After a service-change, a decoder must wait for an I-frame before the decoding process can begin.
Notably, I-frames require include more data and are thus transmitted less frequently than P-frames or B-frames. Typically, a compressed video stream includes two I-frames per second (i.e., once every 500 milliseconds). There is a trade-off between the number of I-frames per second and data bandwidth. Because I-frames are the largest of the three types of frames, bandwidth is saved by reducing the number of I-frames per second. However, a point of diminishing returns in reached, since the longer the time between I-frames, the more the picture fails to be predicted and thus more information will be contained in the P- and B-frames. Since a decoder must wait for an I-frame to start decoding, as the number of I-frames per second is reduced, the channel-change time is increased. For example, for a compressed video stream having two I-frames per second, the channel-change time may be as long as 500 milliseconds. Compared to conventional television, this delay is undesirable to a viewer.
Accordingly, there exists a need in the art for a method and apparatus for distributing compressed video to subscriber terminals that reduces the delay in acquiring new channels.